Photochemical image forming reactions utilizing substituted bis-aryl ethylenic compounds have been described in the following U.S. Patents:
U.S. Pat. No. 3,510,304; PA0 U.S. Pat. No. 3,533,792; PA0 U.S. Pat. No. 3,573,911; PA0 U.S. Pat. No. 3,764,334; PA0 U.S. Pat. No. 3,769,023; PA0 U.S. Pat. No. 3,986,880. PA0 B. where R.sub.1 and R.sub.2 may be any one of Group A and neither is H, either identical with each other or different. PA0 C. where R.sub.1 is H and R.sub.2 may be NZ.sub.2 where Z is H or alkyl (C.sub.1 to C.sub.4 as in A), H and alkyl (C.sub.1 to C.sub.4 as in A), or benzyl (CH.sub.2 Ph) and the two R.sub.2 groups may be either identical with each other or different. PA0 D. where R.sub.3 and R.sub.4 may be H, alkyl (C.sub.1 to C.sub.4 as in A), or phenyl, either identical with each other or different.
These utilize principally as the color formers, 1,1-bis-(p-dimethylaminophenyl)ethylene and its congeners. An exception is U.S. Pat. No. 3,573,911 in which the analog of the ethylene substituent is a thioketone whose proper chemical designation is 1,1-bis-(4-dimethylamino)thiobenzophenone. In all these patents, the principal free-radical activators used as a free-radical source are organic halogen compounds represented by the general formual A-C-X.sub.3, wherein each X represents an iodine, bromine or chlorine atom and not all the X's need to be the same, and A represents a monovalent substituent taken from the group consisting of H, Cl, Br, I, alkyl, aryl, aroyl, and the like.
Of particular interest for the purposes of this presently disclosed invention is U.S. Pat. No. 3,533,792 in which the single dye former 1,1-bis-(p-dimethylaminophenyl)ethylene was used as the principal color former and in which specialized additives were made thereto so that an appropriate mixture of red, blue and green colors could be achieved by color coupling in the heat fixing step to yield an essentially black or near-neutral image specifically through the use of iodoform as the organic halogen compound acting as the source of free-radicals so that the described system could serve for photographic purposes.
The referred to patents define various additions for improvement of shelf life, increase in photographic speed, contrast and the like, through the use of such compounds as triphenylstibine, substituted phenols and cresols, N-oxides, bis-cyclic nitrogen compounds, high molecular weight alcohols, plasticizers, speed enhancers such as triphenylcarbinol and the like and mixtures thereof in order to yield practical systems of reasonable photographic speed, shelf stability, contrast and optical absorption suitable for practical purposes.
In addition, speed increases through use of optical development prior to heat fixing were described generally as a consequence of blanket exposure of the previously exposed system, in which the initial image-wise exposure was sufficient to produce either a barely visible or latent image and in which the wavelength of blanket exposure was equal to the spectral absorption peak of the barely visible or latent image produced as the result of the prior exposure, the two wavelengths being widely separated. Thus, ideally, the light amplifying step is most effective when the system is primarily U.V. sensitive and the principal light absorption peaks of the latent or near latent image produced in the image-wise expsoure resides at wavelengths at least 200 nm of longer wavelength than the long wavelength absorption edge of the moiety initially sensitive to the exposing light.
All of the compositional variations needed to produce the desired characteristics as defined in the patents indicated are applicable in the present invention.
While this broad base of activators and sensitizers can be used for stabilized image production in general, the use of iodoform of formula CHI.sub.3 was found to be the most effective activator for the bis-aryl vinylidene systems, taking all aspects of practicality into consideration.
Thus, of particular background interest for the purposes of this invention with regard to the type of color formation which can be obtained as a consequence of exposure to light followed by heat fixing with or without an intervening optical development step for speed enhancement purposes, are the various patents in the foregoing list which emphasize iodoform in the examples as the principal activator.
In examining the various systems as disclosed by the prior art referred to using iodoform as the activator, practical experience has defined that the normal dry film photosensitive layer which is obtained and needed to achieve maximum density and appropriate keeping qualities falls in the range of 5 to 9 microns.
Certain applications for these types of image forming photosensitive layers involving readout of the optical information imposed on such films as derived by contact printing with a suitable master, defines that maximum resolution along with high edge acuity of image and with adequate optical density of image can be obtained only in photosensitive image producing layers which have a thickness of 3 microns or less. The 3 micron limitation is imposed by the limitations of the optics required for microscopic type of focused beam readout.